The aim of the present study was to investigate whether canine plasma fibrinogen concentration changes with age. We also investigated sex differences in the plasma fibrinogen concentration. As a result, the plasma fibrinogen concentration was found to moderately increase with age (Spearman's r = 0.55, P < 0.001), whereas that of the male and female dogs did not differ to a statistically significant extent. This finding suggests the need to consider aging as a factor that affects the plasma fibrinogen concentration.